CARS - Coherent Anti-Stokes Raman Scattering

CARS measurement on an atmospheric rich quench lean combustion chamber at the institute of propulsion technology.

In real combustion chambers mixing and residence time of the reactants at specific pressures and temperatures are of major importance for the combustion efficiency and the reduction of pollutants.
Broadband-CARS (publications 1, 2) can provide non-intrusive, time and space resolved point measurements of static gas temperature and molecular concentrations in combustion chambers. The measured PDF´s (probability density functions) contain information on the mixing of the combustion chamber flow and the time development of the energy conversion. To produce a CARS-signal three pulsed laser beams of different frequences are focused in the medium investigated and superposed at the measurement location. By the interaction of the electromagnetic fields with the combustion gases a blue-shifted pulsed signal beam is generated which containes information on the gas condition. The evaluation of temperature or concentration is based on the analysis of the spectral resolved intensity distribution of the scattered light, which is called a CARS spectrum. As a consequence of the temperature dependent change of the internal molecular energy the contour of the CARS spectra changes in a characteristic way with the temperature. CARS is used since the seventies (publications 1-3) and described in several reviews (publications 4-6). The measurement technique was successfully applied in low pressure, atmospheric and high pressure environments (publications 7).

Experiment Set-up

CARS Signal

For broadband BOX-CARS typically a spectrally broad dye laser and two narrowband green laser beams are used. Therefore a complete CARS spectrum is generated in a single "CARS laser pulse". Pulse durations are typically 10 to 15 ns and enable time resolved measurements and the acquisition of PDF´s.